How Hawking is still alive?

On April 20, 2009, a moment arrived that doctors had foretold for decades. Stephen Hawking, a scientist who overcame debilitating disease to become the world's most renowned living physicist, was on the cusp of death. The University of Cambridge released grim prognoses. Hawking, diagnosed with amyotrophic lateral sclerosis (ALS) at the age of 21, was described as "very ill" and "undergoing tests" at the hospital. Newspapers ran obituary-esque articles. It seemed time was up for the man who so eloquently explained it.

But, as is his custom, Hawking survived.

Hawking shouldn't be able to do the things he now does. The 73-year-old shouldn't be able to deliver meditations on the existence of God. He shouldn't be able to fret over artificial intelligence or humanity's capacity for self-destruction. And he most definitely shouldn't be able to attend the BAFTAs — Britain's academy awards — settled inside the wheelchair that has carried him for decades, expressing admiration for a recent biopic that paid homage to his struggle. But yet, he is. And he does.

It's difficult to overstate the lethality of ALS, the condition with which Hawking lives. The disorder can befall anyone. It first brings muscle weakness, then wasting, then paralysis, ripping away the ability to speak and swallow and even breathe. The ALS Association says the average lifespan of someone diagnosed with the condition is between two and five years. More than 50 percent make it past year three. Twenty percent make it past year five. From there, the number plummets. Less than 5 percent make it past two decades.

And then there's Hawking. He has passed that two-decade mark twice — first in 1983, then in 2003. It's now 2015. His capacity for survival is so great some experts say he can't possibly suffer from ALS given the ease with which the disease traditionally dispatches victims. And others say they've simply never seen anyone like Hawking.

So what makes Hawking different from the rest? Just luck? Or has the transcendent nature of his intellect somehow stalled what seemed an imminent fate? No one's quite sure. Even Hawking himself, who can expound at length on the mechanics that govern the universe, is circumspect when it comes to an accomplishment that rivals his academic triumphs. "Maybe my variety [of ALS] is due to bad absorption of vitamins," he said.

Hawking's battle with ALS was different from the beginning. And those differences, scientists say, partly explain his miraculous longevity. The onset of ALS normally occurs later in life — the average age of diagnosis is 55 — but Hawking's symptoms materialized when he was very young when he was bright student at Oxford.

Though the early diagnosis resigned him to a life of sickness, it also granted him a chance at surviving the disease longer than those who are diagnosed much later. But still the mystery remains like all his works.